Coating devices are well known and are widely used to apply layers of materials to webs and also to form the webs themselves. Commonly, coaters employ a roll and a beam, or two rolls to form a gap through which a coating of a prescribed thickness may be produced. Examples of such coaters are roll coaters, knife coaters, and reverse roll coaters. Other coaters employ a slot orifice method wherein coating fluid is dispensed from a coating head in the form of a stream. One such slot orifice coater is a bead-coater, which is commonly used in the photographic industry. Slot orifice coaters use a backup roll to support the web as it travels past the coating head.
Control of the width and shape of the gap between the roll and beam or the two rolls is important to producing useable coatings on both gap coaters and slot orifice coaters. In order to ensure that the thickness and uniformity of the web or coating does not vary beyond certain pre-set parameters, it is desirable to be able to adjust the size and shape of the gap to compensate for variations which may result over time (e.g. changes in viscosity, flow, temperature, and web speed, and wear of the die and the mechanical parts of the device).
Numerous mechanisms are known to adjust the size of the gap between a beam and roll. One common mechanism uses helically threaded bolts which, when rotated, move a wedge placed between the bearing mounting of the roll and a structural extension of the beam. Because a force is applied to hold all three of these members together in physical contact, movement of the wedge changes the spacing between the beam and the roll. Another method employs threaded bolts directly. The bolts are threaded through a structural extension of the beam and their ends bear against the roll bearing mounting. Force is applied to hold the bolt end against the bearing mount. Rotation of the bolt directly changes the spacing between the beam and the roll. Still another method involves having a flexible beam rigidly mounted at only one or a limited number of points or edges, and placing a plurality of bolts which can bear against and apply force to bend the beam so as to effect adjustment of the size of the gap between the roll and the setting edge of the beam.
Traditionally, the size of the gap has been adjusted by manually tightening or loosening the bolts with a wrench. It is also known to use individual heaters to heat the bolts in order to cause the length of the bolts to expand, thus changing the size of the gap.
It is also known to use piezoelectric and magnetostrictive translators to adjust the size of the gap. Typically, a measuring device is located downweb of the coater and sends signals to the piezoelectric translators to adjust the size of the gap. These apparatus are based on the assumption that the thickness of the coating at the location of the measuring device is the same as the thickness of the coating at the gap. While this assumption is generally true for slowly evolving changes, it is not true for more rapid changes in coating thickness-those variations which appear and disappear so quickly that the coating thickness being measured at the sensor does not represent the coating thickness at the gap. Indeed, for rapidly repeating variations in coating thickness, the control system may actually exacerbate the problem. For example, if the downweb distance between the measuring device and the gap is equal to the distance between an area of thinner and thicker coating on the web caused by periodic variations in coating thickness, then the measuring device would be measuring an area of thicker (or thinner) coating, while the area of thinner (thicker) coating was passing through the gap. Accordingly, the measuring device would signal the piezoelectric actuators to decrease (increase) the size of the gap, even though the gap was already too small (large).
Rapidly repeating variations in coating thickness are caused by many phenomena, including periodic variations in the thickness of the web caliper upon which the coating is placed, periodic changes in coating fluid viscosity and roll speed, and most importantly, periodic roll "runout," which refers to irregularities in the rotational path of the surface of the roll. Periodic runout occurs because the rolls are not perfectly round, their bearings are not perfectly made, and their supporting shafts are not perfectly straight. Periodic runout is the same for every revolution of the roll.
While the rolls used in coaters may have a radius of about 2 to 50 cm, imperfections as small as 1 .mu.m can result in the uneven application of a coating. Because the roll can rotate at 10 or more revolutions per minute, the use of heat adjustable bolts, which have response times on the order of minutes, to regulate gap size to compensate for periodic runout is impractical. Systems for slowly adjusting the gap with motor driven screws are known but they cannot be effectively used to compensate for roll runout because they have limited speed and accuracy of response. Additionally, the backwards and forwards movements of any mechanical mechanism required by the oscillatory nature of runout compensation creates severe wear and maintenance problems.
It would be desirable to have an apparatus capable of applying a coating that is free from the periodic inconsistencies associated with known coating devices, particularly those apparatus which employ one or more rolls. Any such apparatus must be efficient and accurate to be commercially viable.